1. Field of the Invention
This invention generally relates to a process and apparatus for combustion gas cleaning; and more particularly, relates to a dual alkali process and apparatus for removing sulfur oxides, primarily present as sulfur dioxide, from combustion gases. Relatively small amounts of chlorides can be present with the sulfur oxides in the combustion gas prior to removal of the sulfur oxides. The combustion gases result from a variety of combustion processes generally employing a fuel which has significant sulfur content and less than significant chloride content. To date, the invention has proved useful in cleaning coal-fueled, power-generating-plant, combustion gases prior to the release of the combustion gases to the atmosphere. It can be appreciated that the invention is particularly applicable to the removal of sulfur dioxide from the waste gases of power plants, steam generators, space heating boilers and a variety of manufacturing facilities such as metallurgical smelters, sulfuric acid production facilities and organic sulfonation processes.
As used hereinafter, the term "alkali" is used generically and refers both to "the alkali-metals", a group generally recognized as consisting of the elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) and francium (Fr), and to "the alkaline-earth metals", a group generally recognized as consisting of the elements beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).
2. Description of the Prior Art
It is well known that technology for removal of sulfur oxides from gas streams is quite broad. It is equally well known that removal of at least a portion of sulfur oxides from gas streams is usually done to meet governmentally established environmental procedures. A portion of the existing broad technology relates to a variety of processes which employ several basic steps. A first step generally comprises adding a first chemical to the sulfur oxide bearing gas stream to remove at least a portion of the sulfur oxides from the gas stream. During the first step, the sulfur oxides and first chemical combine to form a second chemical. The first step is generally referred to as the sulfur oxide "scrubbing step" even though the first step need not take place within a scrubber. A second step generally comprises adding a third chemical to the second chemical to cause an easily-removable sulfur-bearing compound to form. Thereupon, certain process equipment operates in a fashion as to cause the sulfur oxides to be physically partially removed from the gas stream in the form of the sulfur-bearing compound. During the second step, a fourth chemical, in addition to the easily-removable sulfur-bearing compound, is formed. The second step is generally referred to as the sulfur oxide "removal step". A third step generally comprises adding a fifth chemical to the fourth chemical to regenerate the first chemical. The first chemical is then re-used in the first step. The third step is generally referred to as the "regeneration step".
Quite often, a process deemed necessary to achieve the scrubbing, removal and regeneration steps outlined above is less than simple. Processes requiring expensive and exotic equipment are as well known as processes requiring complex operating procedures.
Mechanisms useful in the present invention are also well known. The principle mechanisms observed to be occurring in the process equipment of the present invention appear below as chemical equations 1 through 4. EQU Na.sub.2 SO.sub.3 +SO.sub.2 +H.sub.2 O.fwdarw.2NaHSO.sub.3. (1) EQU CaO+H.sub.2 O.fwdarw.Ca(OH).sub.2. (2) EQU Ca(OH).sub.2 +2NaHSO.sub.3 .fwdarw.CaSO.sub.3 .dwnarw.+Na.sub.2 SO.sub.3 +2H.sub.2 O. (3) EQU Na.sub.2 CO.sub.3 +2NaHSO.sub.3 .fwdarw.2Na.sub.2 SO.sub.3 +CO.sub.2 .uparw.+H.sub.2 O. (4)
Equation (1) represents the scrubbing step where an aqueous sodium sulfite (Na.sub.2 SO.sub.3) solution is used to remove at least a portion of sulfur dioxide (SO.sub.2) from a gas stream. Equation (1) also illustrates the formation of sodium bisulfite (NaHSO.sub.3). Equation (2) represents a lime-slaking step where lime (CaO) combines with water (H.sub.2 O) to become slaked lime (Ca(OH).sub.2). Equation (2) is illustrative of introduction of a first alkali (calcium being one of the alkaline-earth metals) into the process equipment of the present invention. Equation (3) represents the removal step where slaked lime combines with an aqueous solution of sodium bisulfite thereby causing a precipitate of calcium sulfite (CaSO.sub.3 .dwnarw.) to form. Equation (4) represents the regeneration step where soda ash (Na.sub.2 CO.sub.3) is added to the aqueous solution of sodium bisulfite to regenerate sodium sulfate. In this regeneration step, some gaseous carbon dioxide (CO.sub.2 .uparw.) evolves as a reaction product. Equation (4) is illustrative of introduction of a second alkali (the soda ash comprising a sodium part and a carbonate part, the sodium part being one of the alkali metals) into the process equipment of the present invention.
It is known that certain sulfur oxides and sulfur dioxide react in a chemically similar fashion. As used herein, the term "sulfur oxides" is used generically in reference to any compound formed from sulfur (S) and oxygen (O). Sulfur dioxide (SO.sub.2) and sulfur trioxide (SO.sub.3), therefore, are specific kinds of sulfur oxides.
It is also known that certain alkali-metal compounds and certain related alkali-metal compounds react in a chemically similar fashion, these certain alkali-metal compounds comprising cationic portions containing one kind of alkali-metal and anionic portions, the related alkali-metal compounds comprising cationic portions containing at least one other kind of alkali-metal and the same anionic portions. It is similarly known that certain alkaline-earth metal compounds and certain related alkaline-earth metal compounds react in a chemically similar fashion, these certain alkaline-earth metal compounds comprising cationic portions containing one kind of alkaline-earth metal and anionic portions, the related alkaline-earth metal compounds comprising cationic portions containing at least one other kind of alkaline-earth metal and the same anionic portions.
As can be appreciated by those skilled in the art, the four chemical equations presented above are illustrative of mere generalizations or simplified tendencies to reaching chemical equilibrium. Such equations typically give no information as to the time required to achieve chemical equilibrium, nor do they give information as to the relative percentage or yield of reactant (any chemical located at the left of the horizontal arrow) that converts into product (any chemical located at the right of the horizontal arrow).
In a process designed to remove sulfur dioxide from combustion gas, the percentage of sulfur dioxide which must be removed is often critical. Elaborate processes requiring significant outlay of capital are well known in the sulfur dioxide gas-scrubbing art.
Desired overall sulfur dioxide removal efficiency is usually the effect of such phenomena as reaction kinetics, chemical equilibrium, thermodynamics, energy of activation, nucleation rate, crystal growth rate, and a variety of other phenomena, all of which are governed by physical laws. These phenomena, in turn, are combined with current economics to define process operating conditions (often referred to as operating parameters) such as operating temperature, solution pH, reactant concentration, reaction residence (or contact) time, and type of reactor for processes designed to remove sulfur dioxide from a sulfur dioxide-bearing gas stream. Usually, overall sulfur dioxide removal efficiency is dependent upon certain additional process equipment being functionally incorporated into an overall process design. Such process equipment is often especially designed to meet specific mixing, leaching, ion-exchanging, classifying, decanting, filtering, screening, centrifuging, or other process requirements. Such additional process equipment often necessarily operates in combination with scrubbing or clarifying equipment, holding tanks or reactors.
Reference is made to U.S. Pat. Nos. 3,911,084; 3,989,797 and 4,147,756 as illustrative of prior art processes and systems over which the present invention provides significant improvements.
It is an object of the present invention to provide an efficient and simple process for the removal of sulfur oxides, primarily present as sulfur dioxide, from waste gas streams.
Another object is to provide a cost-efficient process for removal of sulfur oxides from waste gas streams, the physical process requiring a minimal outlay of capital.
A further object is to provide a process of the above-described type which is an improvement over commercially available processes.
These and other objects and advantages of the present invention will become evident from the description which follows.